Screen grid for power tubes and method of making the same



July 27, 1965 N. B. MEARS 3,197,390

SCREEN GRID FOR POWER TUBES AND METHOD OF MAKING THE SAME Filed July 2, 1962 2 Sheets-Sheet 1 //Yl/E/V7'0R Mae/m4 5. MEHRS Arrows/5) July 27, 1965 N. B. MEARS 3,197,390

SCREEN GRID FOR POWER TUBES AND METHOD OF MAKING THE SAME Filed July 2 1962 2 Sheets-Sheet 2 United States Patent Office 3,1913% Patented July 27, 1965 3,197,390 SCREEN GRID FUR POWER TUBES AND METHGD 61F MAEGNG Tim SAME Norman E. Mears, Dakota County, Minn. (1176 Bobb Road, St. Paul, Mimi.) Filed .l'uly 2, 1962, Ser. No. 206,873 6 Claims. (Cl. 2M1l} This invention relates to improvements in screen grids for power tubes wherein the cathodes may reach temperatures on the order of 8000 C. or higher, and an important factor in the eficiency of electron emission is the capability of the screen gird bars or filaments to remain self-sustaining at temperatures close to the fusion point of the metal from which they are formed.

Heretofore screen grids of this general type have been constructed from wires on the order of .01 inch in diameter spaced apart around the circumference of the grid approximately .01 inch.

It is an object of my invention to provide screen grids which are so constructed as to increase the electron emission from the tube cathode and to provide greater heat resistance and dissipation than the screen grids heretofore designed for like use.

A further object of the invention is to provide an improved method for forming such screen grids which reduces the cost of manufacturing them and makes it feasible to increase the cathode electron emission while maintaining the temperature of the grid within safe limits.

A further and particular object is to provide a method of forming grids for power tubes which includes the steps of electro-forming the cup shaped grid on a matrix, at least a portion of which is removable by heat or chemical treatment, the electro-forrned elements including elongated parallel grid bars connected by integral portions of electro-deposited material at the opposite or upper and lower ends of the grid bars.

My invention also includes certain other novel features of the grid structure and method which will appear and be more fully pointed out in the following specification and claims.

Referring to the drawings:

FIGURE 1 is a top plan view of a suitable matrix on which a screen grid may be formed;

FIG. 2 is a side elevational view of the matrix shown in FIG. 1;

FIGS. 3-7, inclusive, are fragmentary part plan and part sectional views showing portions of the matrix on an enlarged scale, and illustrating successive steps in the method of forming the grid;

FIG. 8 is a fragmentary horizontal sectional view showing a modification of the matrix;

FIG. 9 is a central vertical sectional view showing the modified matrix with electro-formed metal grids on the inner and outer surfaces thereof;

FIG. 10 is a horizontal sectional view taken on the line 1010 of FIG. 9 after the plastic matrix has been removed;

FIG. 11 is a vertical sectional view taken on the line 1111 of FIG. 9, and

FIG. 12 is a fragmentary vertical sectional view showing one of the finished screen grids.

Referring to FIGS. 1 and 2, the illustrated matrix is formed from a thermoplastic, dielectric material which may be liquefied by heat treatment or dissolved in a suitable solvent. The matrix body includes a generally cylindrical upper portion 15 having a flat top and formed with a multiplicity of uniformly spaced, vertically elongated grooves 16 terminating at their lower ends above a conical portion 17 rising from an annular flange 18. A coaxially disposed handle 19 projects from the lower side of the flange 18 to facilitate manipulation of the matrix. As indicated in FIG. 3, the grooves 16 are of uniform width and depth and have sides 16a which extend radially inward from the cylindrical outer surface 16b.

The matrix hereinbefore described is preferably formed by injecting a suitable plastic material into a mold of the required shape and causing or allowing the material to set. it is also feasible to form grooves 16 by machining procedure. Having formed the matrix, the next step in the forming of the screen grid is to apply a thin coat of suitable metal to the surfaces indicated by the broken line 20 in FIG. 2 including the surfaces defining the grooves 16. A liquid emulsion of sliver, for example, may be applied by dipping or spraying the surfaces with a coat of uniform thickness, preparatory to the electrodeposition of another metal on the coated surfaces.

FIG. 4 shows a coat 21 of a first metal covering the surfaces defining the grooves 16 and the surface 16b. As the next step, the coating 21 is removed from the surfaces 1619 and these surfaces are polished, leaving the metal coat on the surfaces defining the grooves 15, as indicated in FIG. 5, and on the entire upper end surface of the cylindrical portion 15, on the conical portion 17, and on the upper surface of the annular flange 18.

These metal coated surfaces are then electro-plated with a second metal in a suitable or conventional man ner. The metal coated matrix is connected in circuit as the cathode in an electro-plating bath and current of suitably low density is employed to insure the formation of a continuous metal plate of the required thickness on the selected surfaces of the matrix. As indicated in FIG. 6, the electro-depositing of metal is continued until grid bars 22 of the required thickness are formed in the grooves 16. To insure the formation of all bars of the required depth radially, the plating may be continued until the bars protrude slightly from the peripheral surface 1611.

A multiplicity of uniformly and circumferentially spawd parallel grid bars are thereby electro-forrned integrally with a generally circular upper end wall 23 and a continuous annular skirt portion 24 (FIG. 12). As indicated in FIG. 7, the projecting portions of the grid bars 22 may then be removed by a polishing operation so that the bars are flush with the cylindrical surface 16b of the matrix. Finally the matrix is removed from the electro-formed screen grid by either heating the matrix to liquefy the portions thereof in contact with the grid elements or by applying a suitable chemical solvent to the plastic matrix.

For tubes requiring screen grids comprising telescoping inner and outer cup-shaped members, a second grid cup like that hereinbefore described may be formed to fit in spaced, telescoping relation to the first grid cup and separated therefrom by spacers preferably formed from a ceramic material.

As an alternate to forming telescoping screen grids on separate matrices, as hereinbefore described, a pair of the screen grids may be formed simultaneously on a modified matrix such as that illustrated in FIGS. 8 and 9. The modified matrix may comprise a tubular body having a generally cylindrical member 25 formed with an outer series of spaced parallel grooves 26 in its outer peripheral surface, and a similar series of grooves 27 formed on its inner peripheral surface and arranged in radial alignment with the several grooves of the outer series. A closed upper end wall 23 is provided on the cylindrical portion of the matrix (FIG. 9), and a skirt portion 29 of plastic material surrounds the lower or open end of the tubular plastic matrix. The end wall 28 is preferably aceramic disk which performs the function of a spacer in the tinished screen grid; Ceramic pins 30 are spaced at suitable intervals around the skirt portion 29 of the matrix 3 and are of such length as to project inwardly and outwardly a distance approximately equal to the thickness of the metal that is to be formed on the inner and outer surfaces of the skirt portion 29.

My method of forming a pair of telescoping screen grids on the matrix shown in FIGS. 8 and 9 is a modification of that described with reference to FIGS. 17. A first metal is applied as a thin coat to the inner and outer surfaces of the matrix members 25, 28, 2) and 3t including the surfaces defining the grooves 26 and 27 and is allowed to set. Thereupon the cylindrical inner and outer surfaces of the member are polished to remove the first metal coat, leaving the coating on the surfaces defining the grooves 26 and 27 and inner and outer surfaces of the members 28 and 29. A second metal is then electroformed on the matrix to till the grooves 26 and 27, thereby forming an outer series of grid bars 31 and a radially inwardly spaced series of bars 32. This electroplating operation also results in the formation of an outer metal disk 33 integral with upper ends of the grid bars 31, an inner metal disk 34, integral with the grid bars 32, and skirt portions 35 and 36, integral respectively with the lower ends of the grid bars 31 and 32. The ceramic pins retain the skirt portions and 36 in spaced concentric relation one to the other.

Since the mounting of the telescoping grid cups does not require the skirt portion 36 to be extended substantially below the cylindrical portion of the matrix, it is unnecessary to extend the electro-formed metal over the inner or lower surface of the matrix skirt portion 29.

In general, the selection of the particular metal to be electro-plated on a matrix for a screen grid is dependent on the conductivity, fusion temperature, vapor pressure, cost and susceptibility of the metal to electro-plating use. Metals such as copper, nickel and chromium are suitable for this purpose. Excellent results have been obtained from electro-formed pure copper screen grids containing traces of one or more hardening elements to provide increased rigidity and ease of handling. Traces of chromium or titanium, for example, in the copper may be employed to impart the desired strength. If a silver solution or emulsion is used to form the first metal coating on the matrix, all silver must be removed from the surfaces of electro-formed second metal because of the high vapor pressure of silver. This cleaning treatment for the removal of silver may be obviated by applying a copper film directly to the matrix preparatory to the electro-plating of copper or other suitable metal on the matrix. Accord- I ingiy, the expressions first metal and second metal as used herein are intended to include successive applications to the matrices of the same or different metals or alloys.

Examples of suitable ceramics for use in forming the spacer disk 29 and pins 30 are fosterite, mullite, steatite and ziron.

Numerous comparative tests have established the superiority of my electro-formed screen grids over the stand ard screen grids which have heretofore been formed from fine wire. In one series of tests copper electro-forrned grids, made according to the present invention, were compared with standard grids of equal size in a special fixture for thermal and electrical evaluation. To measure the grid temperature versus wattage, a .002 diameter point to point Rh thermocouple was spot welded to one of the grid bars of each test sample, and the grid was mounted in a suitable structure that permitted the thermocouple wire to be brought through the envelope without creating additional thermocouple junctions. Two structures were built, one containing the electro-formed grid, the other a standard grid formed from Wires of pure molybdenum. A plot of grid temperature versus grid wattage was made through the wattage range 0 to 20. At each point five minutes was allowed for the grid temperature to reach equilibrium. A millivolt bridge was 'used to measure the thermocouple This series of tests established that at the minimum rated grid wattage (12 watts) the electro-plated grid operates approximately 25% cooler than the standard grid, and that the grid temperatures of the electro-formed grid and standard grid, at this wattage, were 585 C. and 620 C. respectively. Thus the electro-formed grid permitted a grid Wattage increase of approximately 8% over an equivalent temperature using a standard grid.

In another series of tests, two vehicles, one containing the electro-formed grid and the other the standard screen grid, were operated in a laboratory grid emission unit at powers ranging from 6 to 20 watts. The primary emission was read and recorded. The power on the grid was increased in 2 watt steps at intervals of 2 minutes and the emission read at the end of each 2 minute interval. The vehicle with the standard grid was operated at 6.8 heater volts and the one containing the electro-formed grid was operated at 7.0 volts for the reason that at these voltages the cathode temperatures in the two vehicles were the same, viz.: approximately 8000 C. ET. The plots indicated that the emission from the electro-formed grid was greater than that from the standard grid for reasons unknown. This second series of tests included a test run in which emission readings were taken after a series of 15 minute standby periods, and another run in which readings were taken after sixty minute standby intervals. At the 12 watt minimum grid rating the electro-formed grid showed a 250 a. reading after a 15 minute cathode standby period and a ,aa. reading after the 60 minute standby. This effect of increased standby was reversed in standard grids which showed at the 12 watt power rating 8 ,ua. after the 15 minute standby and 22 a. after the 60 minute standby.

It will be evident that screen grids produced according to the present invention are superior to those heretofore known in a number of respects, and that my method reduces the cost of producing uniformly superior grids.

I claim:

1. A method of forming a screen grid for electronic tubes comprising:

(a) forming a matrix of dielectric material having a substantially circular end, an elongated substantially cylindrical portion, a multiplicity of spaced parallel grooves extending longitudinally of said cylindrical portion, and an annular skirt portion;

(b) applying a first coat of metal to said end, cylindrical, and skirt portions, and to the surfaces defining said grooves;

(c) removing said first coat of metal from said cylindrical portion of said matrix, leaving the metal coating on said end and skirt portions and on the surfaces defining said grooves;

(d) electrodepositing a metal layer on the remaining metal coated surfaces of said matrix;

(e) continuing the electrodeposition of the said metal layer until said grooves are filled to the face of said cylindrical portion and to form a screen grid of the desired thickness, and

(f) treating said matrix to remove at least portions thereof from the electroformed grid body.

2. A method of forming a screen grid in accordance with claim 1 in which said substantially cylindrical portion of said matrix is formed from a thermoplastic material and is heattreated to a molten condition for removal from said electroformed grid body.

3. A method of forming a screen grid in accordance with claim 1 in which said matrix is formed at least in part from a chemically reactive plastic material adapted to be liquefied by chemical treatment, and including the step of removing said plastic matrix from the electroformed metal body by chemical dissolution.

4. A method of forming a screen grid of cup shape for electronic tubes comprising:

(a) forming a matrix of dielectric material comprising,

a tubular body having an open end and a closed end,

an end wall defining inner and outer end surfaces, an elongated substantially cylindrical Wall portion, a multiplicity of spaced parallel grooves extending longitudinally of the inner and outer surfaces of said cylindrical wall portion, and a skirt portion surrounding the open end of said tubular matrix;

(b) applying a thin coat of metal over the inner and outer surfaces of said end wall and skirt portion and on the inner and outer surfaces of said cylindrical Wall portion, including the surfaces defined by said grooves;

(c) removing said coating of metal from the cylindrical, inner and outer surfaces of said cylindrical portion of the matrix, leaving said thin metal coating on said end wall and skirt portions and on the surfaces defining said grooves;

(d) electroplating a second metal on the remaining metal coated surfaces of said matrix;

(e) continuing the electrodeposition of said second metal until said grooves are filled to the inner and outer faces respectively of said cylindrical wall portion of the matrix, and

(f) treating said matrix to removes at least the cylindrical wall portions thereof from the electro-formed metal body.

5. A method of forming a screen grid in accordance with claim 4 in which said first coat of metal is applied to a thickness within the range .0001 inch.00l inch and in which the electrodeposition of said second metal coat is continued until the grid cup has a thickness within the range .003 inch-.05 inch.

6., A method of forming a screen grid cup in accord- References Cited by the Examiner UNITED STATES PATENTS 2,650,900 8/53 Holman 204--11 2,702,270 2/55 Donahue et a1 20411 3,028,516 4/62 Foote et a1 3l3-297 X JOHN W. HUCKERT, Primary Examiner.

GEORGE N. WESTBY, Examiner. 

1. A METHOD OF FORMING A SCREEN GRID FOR ELECTRONIC TUBES COMPRISING: (A) FORMING A MATRIX OF DIELECTRIC MATERIAL HAVING A SUBSTANTIALLY CIRCULAR END, AN ELONGATED SUBSTANTIALLY CYLINDRICAL PORTION, A MULTIPLICITY OF SPACE PARALLEL GROOVES EXTENDING LONGITUDINALLY OF SAID CYLINDRICAL PORTION, AND AN ANNULAR SKIRT PORTION; (B) APPLYING A FIRST COAT OF METAL TO SAID END, CYLINDRICAL, AND SKIRT PORTIONS, AND TO THE SURFACES DEFINING SAID GROOVES; (C) REMOVING SAID FIRST COAT OF METAL FROM SAID CYLINDRICAL PORTION OF SAID MATRIX, LEAVING THE METAL COATING ON SAID END AND SKIRT PORTIONS AND ON THE SURFACES DEFINING SAID GROOVES; (D) ELECTRODEPOSITING A METAL LAYER ON THE REMAINING METAL COATED SURFACES OF SAID MATRIX; (E) CONTINUING THE ELECTRODEPOSITION OF THE SAID METAL LAYER UNTIL SAID GROOVES ARE FILLED TO THE FACE OF SAID CYLINDRICAL PORTION AND TO FORM A SCREEN GRID OF THE DESIRED THICKNESS, AND (F) TREATING SAID MATRIX TO REMOVE AT LEAST PORTIONS THEREOF FROM THE ELECTROFORMED GRID BODY. 